1. Field of the Invention
The present invention relates to a mold clamping force adjustment device of an injection molding machine having a toggle type mold clamping system.
2. Description of the Prior Art
In an injection molding machine having a toggle type mold clamping system, a toggle mechanism of the toggle type mold clamping system is built between a movable platen and a rear platen. When closing the mold, first of all a movable mold half and a fixed mold half are caused to touch each other in a contraction state of the toggle mechanism. Then, the toggle mechanism is stretched and a tie bar which couples a fixed platen to the rear platen is stretched. Under such condition in which the toggle mechanism is stretched completely (“lock-up state”), a set mold clamping force is generated between the movable mold half and the front mold half by the reaction force (elastic force) of the stretched tie bar.
As above, a mold clamping force is determined based on the amount of stretch of the toggle mechanism from the time when the toggle mechanism is contracted to cause the mold halves to touch each other until the time when the toggle mechanism is stretched completely and applies the set mold clamping force to the mold halves (i.e. the amount of movement of the movable platen). Therefore, it is necessary to adjust the position of the rear platen (mold clamping force adjustment) so that the toggle mechanism is contracted as much as the amount corresponding to a desired amount of stretch to cause the mold halves to touch each other.
Examples of the conventional technologies of such mold clamping force adjustment will now be described hereinafter.
(1) Mold Clamping Force Adjustment Described in Japanese Patent Application Laid-Open No. S62-220314:
A toggle mechanism of a toggle type mold clamping system is contracted as much as the amount necessary to generate a mold clamping force that is set. A torque limit is applied to a servomotor which drives the toggle type mold clamping system, and a geared motor which drives a rear platen is rotated normally. Since an output torque of the servomotor is limited, when the mold halves touch, the toggle mechanism is contracted, and the servomotor is rotated, whereby the value of the error resistor (error value) is increased. When the error value reaches a set value, the drive of the geared motor is stopped. Thereafter, the geared motor is rotated backward, the rear platen and the movable platen (movable mold half) are moved backward, and the mold is opened. Then the movable platen is moved forward as much as the amount of the error value. Furthermore, the torque limit of the servomotor is canceled, the geared motor is rotated normally for a predetermined period of time, and the rear platen and the movable platen (movable mold half) are moved forward. By determining the position of the rear platen as above, the mold clamping force is adjusted.
(2) Mold Clamping Force Adjustment Described in Japanese Utility Model Examined Publication No. H7-32024
There is provided a thermal relay for detecting excess current of a motor for mold thickness adjustment which drives a toggle support of a toggle type mold clamping system. The toggle mechanism of the toggle type mold clamping system is caused to move forward in a state where a set mold clamping force is contracted by the necessary amount to generate. When mold touch occurs, the thermal relay detecting the excess current of the motor for mold thickness adjustment is turned off. This is detected to stop the drive of the motor for mold thickness adjustment and to finish mold thickness adjustment.
(3) Mold Clamping Force Adjustment Described in Japanese Patent Application Laid-Open No. H10-278084 and Japanese Patent Application Laid-Open No. 2001-239562:
The amount of stretch of a tie bar at the time when a toggle mechanism of the toggle type mold clamping system is driven to cause mold touch by means of certain pressing pressure is determined in advance, the amount of stretch of the tie bar is corrected, and the position of the rear platen is determined.
As described above, the toggle type mold clamping system generates a set mold clamping force by using the movement of the movable platen from the time when the mold is touched until the time when the toggle mechanism is brought to the lock-up state. To that end, the position of the movable platen at the time when the mold is touched needs to be detected accurately. If the tie bar is stretched at the time when it is detected that the mold is touched, thereby generating a mold clamping force, the mold clamping force to be generated is increased more than the set mold clamping force by that much.
In the case of the conventional technologies (1) and (2), a mold clamping force is already applied to the mold when mold touch is detected. Therefore, by using the toggle mechanism to move the movable plate as much as the amount necessary to generate a set mold clamping force, the amount of a mold clamping force which is actually generated is larger than the set mold clamping force.
In the case of the conventional technology (3) above, on the other hand, the position of the rear platen is determined so that a set mold clamping force is obtained by correcting the stretch of the tie bar that occurs at the time of mold touch. However, the motor for driving the toggle mechanism is driven in order to cause mold touch so as to obtain the amount of correction, thus a pressing force (mold clamping force) to the mold that is generated varies depending on the bending state of the toggle mechanism. Consequently, this correction amount varies depending on the thickness of the mold, thus it is necessary to measure this correction amount for every mold clamping force adjustment performed in accordance with the mold thickness, which requires time and effort in a mold clamping force adjustment work.